Behavior and Energy State of Photogenerated Charge Carriers in Single-Crystalline and Polycrystalline Powder SrTiO₃ Studied by Time-Resolved Absorption Spectroscopy in the Visible to Mid-Infrared Region

Abstract

The effects of defects on the behavior of photogenerated charge carriers in SrTiO3 (STO) are studied by time-resolved absorption spectroscopy from the visible to mid-IR region. In the case of defect-free single-crystalline STO, free and shallowly trapped electrons are dominant, but they recombine within 50 ns. By contrast, in the case of defect-rich powder STO, the electron lifetime is much longer than 1 ms. The transient absorption spectra show that most of the charge carriers in powder STO are trapped in the defects, which elongates their lifetime. We found that these trapped carriers are nevertheless reactive toward O2 or CH3OH that depends on the trap depth. The steady-state photocatalytic activity is strongly correlated with the lifetime and the reactivity of the trapped charge carriers: the energy state of electrons can be deduced from the spectral shape, especially in the mid-IR region.

Morphology-sensitive trapping states of photogenerated charge carriers on SrTiO3 particles studied by time-resolved visible to Mid-IR absorption spectroscopy: The effects of molten salt flux treatments

“…A clear absorption peak is observed around 11,000 cm À1 . The spectral shape is totally different from that for other commercial SrTiO 3 powders purchased from Aldrich and Kojundo Co., which are already reported in our previous paper [11]. In the case of Aldrich and Kojundo powders, three peaks appear at 20,000, 11,000 and 2500 cm À1 , which are assigned to trapped holes, trapped electrons, and shallowly trapped electrons, respectively.…”

“…Time-resolved absorption measurements were performed with laboratory-built spectrometers as described in our previous papers [10,11,28]. Briefly, in the mid-IR region (6000-1000 cm À1 ), the probe IR light was focused on the sample and the transmitted light was monochromated by the spectrometer.…”

“…Time-resolved absorption spectroscopy in the visible to mid-IR region [10,11] is a powerful method for elucidating the behavior of photogenerated charge carriers, since free electrons [12][13][14][15][16][17][18][19][20] in the conduction band (CB) give rise to structureless broad absorption in the mid-IR region. Trapped electrons and holes in mid-gap states also give transient absorption peaks in the visible or near-IR (NIR) regions [21][22][23][24][25][26][27]; therefore, the energy states of photogenerated charge carriers as well as the decay processes can be examined.…”

“…Furthermore, most of the electrons in LaTiO 2 N are deeply trapped in the mid-gap states but the CoO x cocatalyst decreases the depth of the electron traps. This method was also used to investigate the behavior of charge carriers in powder and single-crystalline SrTiO 3 [11]. We found that free or shallowly trapped electrons are dominant in single crystals, while most charge carriers are deeply trapped in the case of powder particles.…”

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO 2 reduction

“…Transient absorption (TA) spectroscopy has been used extensively to understand the processes that govern the relaxation dynamics of photoexcited electrons and holes and has been successfully applied to the study of midgap states in metal oxides (21,22,26,27,(30)(31)(32)(33)(34). By studying highly defected materials and modeling the TA decay as a function of defect concentration, time, and excitation wavelength, a more detailed understanding of the dominant charge carrier relaxation processes can be obtained (22)(23)(24).…”